FIG. 10 is a perspective view illustrating a conventional infrared imaging device. The infrared imaging device 90 includes a two-dimensional array of photodiodes 1, for example, 128.times.128 photodiodes, for generating electric signals in response to infrared light 4 incident on the respective photodiodes. The photodiode array is mounted on and electrically connected to a silicon substrate 2 containing signal processing circuitry. Each photodiode is in electrical communication with a respective signal processing circuit in the substrate 2 through a columnar body 3, such as a cylindrical volume of indium.
The general electrical arrangement of the imaging array 90 is schematically illustrated in FIG. 11 for a 3.times.3 array of photodiodes 1ato 1i. The signal processing circuitry in the substrate 2 includes a horizontal CCD 2b to which three vertical CCDs 2a are connected. An output circuit 5 is connected to the horizontal CCD 2b.
A description is given of the operation.
Incident infrared light 4 causes the photodiodes 1a to 1i to produce electrical charges that are conducted through the connectors 3 to the respective signal processing circuits in the substrate 2. The signal processing circuit stores the electrical charges and eventually transfers them in a timed sequence through the vertical CCDs 2a to the horizontal CCD 2b. The horizontal CCD 2b further transfers the stored electrical charges in a timed sequence through the output circuit 5 to the outside. In the detection of infrared light having wavelength of about 10 microns, the two-dimensional photodiode array 1 may employ Cd.sub.0.2 Hg.sub.0.8 Te.
FIG. 8 is a plan view illustrating a photodetector employing Cd.sub.0.2 Hg.sub.0.8 Te, and FIG. 9 is a sectional view taken along line IX--IX.
In these figures, a Cd.sub.0.2 Hg.sub.0.8 Te layer 8 is disposed on a CdTe substrate 9. Reference numeral 11 designates a light responsive region. In the light responsive region 11, a two-dimensional array of p-n junctions 10 serving as pixels is disposed within the Cd.sub.0.2 Hg.sub.0.8 Te layer 8, and n side electrode pads 13 are disposed on the respective p-n junctions 10. P side electrode pads 14 are disposed on a p side electrode metal 15. The p side electrode metal 15 is disposed on the Cd.sub.0.2 Hg.sub.0.8 Te layer 8. A TEG 12 for monitoring the characteristics of the p-n junctions 10 in the light responsive region 11 is disposed on the same surface as the light responsive region 11 but outside the region 11. The TEG 12 consists of p-n junctions 10a disposed within the Cd.sub.0.2 Hg.sub.0.8 Te layer 8, n side electrode pads 13a disposed on the respective p-n junctions 10a, and a p side electrode pad 14a disposed on the p side electrode metal 15. The p-n junctions 10 in the light responsive region 11 and the p-n junctions 10a in the TEG 12 are produced at the same time. An insulating film 16 that is transparent to infrared light, such as ZnS, is selectively disposed on the Cd.sub.0.2 Hg.sub.0.8 Te layer 8 to separate the electrode pads 13, 13a, 14, and 14a from each other. Although the light responsive region 11 shown in FIG. 8 includes 3.times.3 pixels 10 for simplification, the light responsive region usually includes more pixels, for example, 128.times.128 pixels.
The TEG 12 is for monitoring characteristics of the p-n junctions 10 in the light responsive region 11. More specifically, probes of a probe card are applied to the p side electrode pad 14a and one of the n side electrode pads 13a to measure I-V characteristics of the p-n junction 10a opposite the n side electrode pad 13a, whereby characteristics of the p-n junctions 10 in the row corresponding to the probed p-n junction 10a are measured. This characteristic measurement using the TEG 12 is carried out after the wafer process, i.e., after fabricating a plurality of photodetectors on a wafer, and then the photodetectors on the wafer are sorted into defective photodetectors and non-defective photodetectors according to the results of the measurement. Thereafter, the indium connectors 3 are formed on the n side electrode pads 13 and the p side electrodes pads 14 of a non-defective photodetector, and the photodetector is flipped over and mounted upside down on the silicon CCD substrate 2, completing the infrared imaging device of FIG. 10.
However, from the measured I-V characteristics of the TEG 12, only the presence or absence of the p-n junction 10 in the light responsive region 11 is detected. Even if a photodetector has insufficient resolution caused by an excessive diffusion length of the minority charge carriers in the semiconductor layer 8 in which the p-n junctions 10 are formed, the insufficient resolution is not detected from the I-V characteristics of the TEG 12, and this photodetector is selected as non-defective. Therefore, even an infrared imaging device including a photodetector selected as a non-defective sometimes forms an indistinct image or, in the worst case, forms no image.
In order to solve the above-described problems, it is thought that the characteristic measurement of the photodetector should be carried out using a conventional resolution measuring apparatus.
FIG. 12 is a schematic diagram illustrating a resolution measuring apparatus. The resolution measuring apparatus 110 consists of a Dewar 54 for containing and cooling an imaging device 1a, an optical system 52 for collecting infrared light 4 from an infrared light source 51 and irradiating the imaging device 1a in the Dewar 54 with a spot of light, an amplifier 56 for detecting the light output from the imaging device 1a, and an oscilloscope 55 for displaying the light output on a screen. Reference numeral 53 designates a chopper for periodically interrupting the continuous infrared light 4. Reference numeral 54a designates a cooler for cooling the Dewar 54, which is filled with liquid nitrogen.
In operation, the light produced by the infrared light source 51 and the optical system 52 is applied to the light responsive region of the infrared device, and the light output from the infrared device is measured by the oscilloscope 55. The spatial resolution of the light responsive region is evaluated by the position of the light and the output voltage from the p-n junction in the light responsive region. However, it is impossible for the Dewar 54 to contain a large-sized wafer including a plurality of photodetectors 1. In addition, the space between the optical system 52 and the Dewar 54 is not variable. Therefore, it is impossible to evaluate the resolution of the photodetector on the wafer by irradiating the photodetector with the light using the apparatus of FIG. 12.